During preparation, perishable items are often held for extended periods of time in open, easy-to-access containers. For example, many kitchens keep condiments and other frequently used foods in trays that are left open to the warm kitchen environment. As a perishable sits in ambient air, however, it absorbs heat through the surface exposed to the environment by the uncovered top of the tray. This heat absorption causes the perishable's internal temperature to rise, eventually warming the perishable to the temperature of the ambient air. In an attempt to counteract this warming process, trays holding perishables are often placed into “condiment rails” or other similar container systems that are designed to remove the heat added by the environment.
A refrigerated condiment rail is a bin that holds a number of individual, perishable-holding trays cooled using a cooling mechanism. This mechanism, which can range from a simple ice bath to a complex arrangement of refrigeration units, such as a “Bloomington Rail”, conductively chills the bottom and side surfaces of the trays. The difference in temperatures creates a temperature gradient between the warmer exposed, perishable surface and the colder tray surfaces. The temperature gradient causes the perishable body to pass the absorbed heat from the exposed perishable surface to the surfaces of the tray (down the temperature gradient). The speed of this transfer is a function of the characteristics of the perishable and the size of the temperature difference. The colder the system is able to make the tray surface, the more quickly the heat can be passed down the gradient. Many perishables become damaged if they are frozen, however, so systems typically keep tray surfaces above −32° F. in order to avoid damaging portions of the perishable near the bottom and sides of the tray.
Systems such as Bloomington Rails typically use conduction to chill the surfaces of the tray, but other methods for chilling the tray surfaces are also used. For example, some systems incorporate forced convection mechanisms that cool the surfaces of a tray by blowing chilled air across them. While forced convection is often less efficient than conduction for the transfer of heat, it is still easily capable of lowering the tray surfaces to the freezing point. In addition, some forced convection systems reduce the temperature of the perishable directly by venting chilled air across the exposed surfaces. This rapidly moving cold air will cool the exposed surface of the perishable further removing the heat added by the environment.
Other systems directly chill the perishable by immersing it in a chilled environment. Chest freezers, for example, hold perishables in an open-topped chamber and continually fill the chamber with cold air. The heavier, colder air amasses in the bottom of the chamber slowly building toward the top. Eventually, all the warmer air is forced out of the chamber completely.